A few of the horses I picked up later in the season have been raced to the limits of their conditioning. I don't intend to race them any further this year so what's the optimal training regimen to keep them from deteriorating until next year's schedule? Low or mid intensity training? Is there a cumulative effect on a horse long term over its career from over-racing, other than the obvious susceptibility to injury?

That's secondary. Every time the horse races or do a manual training gallop, the horse lose long-term condition (red bar in condition). When the bar is very low, it means he is not able to run in that season.
Red training is commonly used during season. But, a good option to keep them in longer term, race every 15-30 days, and put training in blue at least 3 days after a race. Then go red again.

The way it decreases is conditioned by its constitution.